This invention relates generally to furnaces and, more particularly, to multipass heat exchangers therefor.
A typical residential furnace has a bank of heat exchange panels arranged in parallel relationship such that the circulating blower air passes between the panels to be heated before it passes to the distribution duct. Each of the panels is typically formed of a clamshell structure which has an inlet end into which the flame of a burner extends to heat the flue gas, an outlet end which is fluidly connected to an inducer for drawing the heated flue gas therethrough, and a plurality of legs or passes through which the heated flue gas passes. In order to obtain the desired high efficiencies of operation, it is necessary to maximize the heat transfer that occurs between the heated flue gas within the heat exchanger passes and the circulating air passing over the outer sides of the heat exchanger panels. Further, there are required performance and durability requirements for the heat exchanger panels themselves.
One requirement is that the internal pressure drop within the heat exchanger panels is maintained at an acceptable level. That is, in order to minimize the inducer motor electrical consumption costs, it is necessary that the pressure drop be maintained at suitable levels.
Durability of the heat exchanger panels is also an important requirement. In order to obtain long life, the heat exchanger panels must be free of excessive surface temperatures, or hotspots, and the thermal stresses must be minimized. Further, the need for expensive high temperature materials is preferably avoided.
A more recent requirement is that of reducing the height of the heat exchanger panels. This is important for a number of reasons. First, it allows the overall height of the furnace to be reduced such that it can be placed in smaller spaces, such as in attics, crawl spaces, closets and the like. Secondly, it allows for a reduction in costs, both in the costs of the heat exchanger panels themselves and in the cost of the furnace cabinet. But this reduction in height must be done without sacrificing performance. That is, a simple reduction in height, with a proportionate reduction in performance, would not be acceptable. It is therefore necessary to obtain increased performance for a given length or height of the heat exchanger panels.
It is therefore an object of the present invention to obtain an improved heat exchanger for a furnace.
Another object of the present invention is to reduce the overall height of the heat exchanger in a furnace.
Yet another object of the present invention is the provision in the furnace for reducing the size of the heat exchanger while maintaining performance levels.
Another object of the present invention is the provision for a durable heat exchanger with controlled surface temperatures, reduced hotspots and minimal thermal stresses.
Still another object of the present invention is the provision for a heat exchanger with minimal internal pressure drop.
A further object of the present invention is the provision for a heat exchanger which is economical to manufacture and effective and efficient in use.
These objects and other features and advantages become readily apparent upon reference to the following descriptions when taken in conjunction with the appended drawings.
Briefly, in accordance with one aspect of the invention, the heat exchanger surface area, per unit height of a multipass heat exchanger, is increased by providing wavy cross-sectional shapes in the sides of at least two of the passes. Optimal efficiency is obtained while maintaining the pressure drop within the panels at an acceptable level by having the number of waves in the downstream pass being equal to or greater than those in the upstream pass. In this way, high-efficiency heat transfer performance is obtained, while minimizing the flueside pressure drop and the operating costs of the inducer.
In accordance with another aspect of the invention, the wavy shapes are generally sinusoidal in shape, and each side may extend inwardly to or beyond a common central plane.
By another aspect of the invention, there is a single pass in which the cross-sectional shape transitions from a non-wavy shape to a wavy shape. This transition section is of a substantial length, such that the transition from one shape to the other is gradual, thereby providing for reduced temperatures and stresses in that section.
In accordance with another aspect of the invention, a gooseneck shape is provided in the last passage, such that, as the passage approaches the outlet, it curves downwardly toward the second to last passage so as to result in a lower overall height of the heat exchanger while minimizing the reduction of the cross-sectional area of the flow passage.
By yet another aspect of the invention, the first return bend of the heat exchanger varies in cross sectional area in the direction of gas flow, first increasing and then decreasing, so as to reduce the occurrence of hot spots while avoiding an increase in overall height of the heat exchanger.
In the drawings as hereinafter described, preferred embodiments are depicted; however, various other modifications and alternate constructions can be made thereto without departing from the true spirit and scope of the invention.